An interconnection network connecting nodes with each other in an IP exchange network or a parallel computer is commonly configured by connecting routers transferring a packet with each other. In a large-scale network, a completely connected network in which all nodes are connected in a one-to-one manner is not used because an introduction cost is high, and a mechanism that controls routers such that no competition occurs in a communication path is not used because a cost is high.
For this reason, in a router, when a plurality of packets input from different input ports are transferred, the plurality of packets are likely to compete with one another for one output port.
Generally, a router selects and transfers one packet when output competition occurs, and includes a buffer that stores the remaining competing packets during that period of time and delays the transfer. However, since a buffer amount is limited, when competition continuously occurs among packets, the buffer is depleted. Further, when the buffer is depleted, it is necessary to discard an input packet.
In order to avoid the depletion of the buffer, some routers include a flow control function of requesting connection destination routers of competing input ports to stop packet transmission when the remaining buffer amount is small. Particularly, in a parallel computer, flow control in which a packet is not discarded by strictly managing the remaining buffer amount is commonly performed.
In the flow control, information representing that the buffer has been depleted by competition of packets is transmitted to a router at the upstream side in the flow. Thus, for a transmission source node that continuously transmits a packet to a communication path in which competition is occurring, the buffer of the router connected to the transmission source node is depleted ultimately, and so transmission stops.
Generally, when a transmitting apparatus connected to a network transfers data to a specific destination, the data transfer is continuously performed during a certain period of time, and a communication path in which the data transfer is being continuously performed remains unchanged. Thus, the competition in the communication path continues during the certain period of time, causing discarding of a packet or depletion of a buffer. A phenomenon in which competition of packets in a communication path significantly reduces a data transfer speed as described above is referred to as congestion.
When congestion occurs simultaneously in a number of communication paths, a phenomenon in which a data transfer speed is severely reduced widely occurs. Particularly, in case of the packet discarding, there are cases where a phenomenon in which a data transfer speed is reduced and so communication is nearly impossible occurs, and such a phenomenon is referred to as congestion collapse.
For example, congestion collapse occurs when a discarded packet is continuously transmitted while congestion is occurring or when all buffers of a communication path of up to a competition point are depleted.
In order to avoid a severe reduction in a data transfer speed by congestion collapse, congestion control is performed.
The congestion control is implemented by congestion detection and data transmission control in a transmission source node. Since influence of congestion spreads in a short period of time, it is desirable to detect congestion at an early stage. The transmission source node commonly uses a response packet from a destination node to detect the occurrence of congestion. For example, in a transmission control protocol/Internet protocol (TCP/IP) protocol, the occurrence of congestion is detected by detecting the fact that a response packet does not arrive during a certain period of time. This uses a characteristic in which an IP exchange network discards a packet when congestion occurs.
Further, in an Ethernet extension standard for a data center such as data center bridging (DCB), converged enhanced Ethernet (CCE), or data center Ethernet (DCE), packet loss is avoided by flow control, and a means for detecting congestion as well as a retransmission timeout is provided. (Ethernet is a registered trademark).
For any of congestion detection, a mechanism of detecting congestion in routers is employed, and notification of congestion is given to a router of a packet transmission source by transmitting a dedicated packet from a router to a transmission source node or including a flag in a response packet to be transmitted from a router.
Here, there are cases in which a buffer of a router has no capacity enough to hold a packet when congestion occurs, and so influence of the occurrence of congestion is extremely large. For this reason, when congestion is once detected, there is a possibility that a data transfer speed has been already reduced in a wide range due to congestion, and thus, congestion control is required to solve congestion instead of preventing congestion.
An initial congestion control scheme of TCP/IP was a slow start of decreasing a data transfer speed of retransmission to be lower than a normal data transfer speed and gradually increasing the data transfer speed up to a normal communication speed in order to avoid the re-occurrence of congestion by retransmission of a packet.
In this scheme, when congestion occurs, a process in which the data transfer speed is first reduced, then the data transfer speed gradually increases, and congestion reoccurs is repeated. For this reason, there is a problem in that network usage efficiency is low.
Further, with the increase in the speed of the IP exchange network or the development of a technique of preventing congestion collapse in which a router randomly discards a packet before a buffer is depleted, a congestion control scheme of converging a band more rapidly has been also used.
For example, in the de facto standard of the congestion control protocol for TCP/IP, a data transmission amount is reduced to half each time of retransmission. The congestion control in which a data transmission amount is reduced at this rate is generally referred to as an additive increase multiplicative decrease (AIMD) scheme. In the AIMD scheme, converging is performed with the amplitude of the data transfer speed smaller than that in the slow start described above. However, there is still loss in network usage efficiency.    Patent Literature 1: Japanese Laid-open Patent Publication No HEI8-56222 A    Patent Literature 2: Japanese Laid-open Patent Publication No 2000-13391 A    Non Patent Literature 1: IETF RFC 2581
In the congestion control performed in the IP exchange network of the related art as described above, the router gives notification of packet discarding or congestion, and suppresses a data transfer speed in the transmission source node. However, in the IP exchange network, since it is considered that a network configuration can be designed independently of a type and configuration of a computer to be connected thereto, a network configuration is irregular, and how much the data transfer speed has to be suppressed is not clear. For this reason, a scheme of changing a data transfer speed and finding an appropriate data transfer speed has been also used, but this control scheme has a problem in that the data transfer speed is unstable and the network usage efficiency is low, and is inappropriate for an interconnection network configured as a regular network.